One of the most common earth moving machines used in the general construction industry is the mechanized shovel. Such digging machines are generally available in two varieties which are known as the "excavator" and the "backhoe," although a "trencher" (sometimes called a DITCH-WITCH) can also be placed in this category. An excavator is generally the largest of these machine types. A simplified drawing of a machine of this type is shown on FIG. 1.
An excavator, generally designated by the reference numeral 10, usually comprises a tracked machine with a pivot between its lower tracked carriage 18 and its cab assembly 20, which provides for side-to-side motion during operation. The digging apparatus generally consists of two extending members called arms, and a bucket 16. The first arm 12 is commonly called the "boom" and the second arm 14 is commonly called the "dipperstick."
A backhoe is generally smaller than an excavator but shares several similarities. A backhoe is generally a rubber tired machine which has its shovel portion on one end of the machine and another bucket on the other end. This second bucket apparatus is similar to a front-loader and is commonly used for moving material instead of digging. Like the excavator, the backhoe has a shovel implement which typically consists of a boom, dipperstick, and a bucket. This type of machine typically has a pivot between the cab portion and the boom arm to provide side-to-side motion while digging.
In operation, an operator of either a backhoe or excavator (hereinafter referred to generally as an "excavator") typically must dig to a particular elevation. If the operator digs too shallow then he must come back and rework the area; if he digs too deep then excessive fill material must be used. In order to determine the elevation in conventional systems, a second person is used to measure elevations for the machine operator. This person would either be using a laser system or an automatic level to determine the current elevation. If an automatic level is used then a third person is required to operate the level. This is further complicated if the digging depth is desired to be sloping, and not dug to a consistent (i.e., level) elevation. In this case, someone must keep track of the distance moved and periodically either add or subtract a certain elevation based on the distance and the desired slope.
Since some of the conventional digging systems in the prior art are so cumbersome and labor intensive, as described above, it would be very desirable to have available a digging system where the excavator operator can check his own digging elevation without the need for another person's help, and without stopping and getting out of the cab. Therefore, there is a need for an elevation indication system for excavators which is low cost, easy to install, is based on an absolute elevation reference, and if possible, provides elevation information while the operator is actually digging, rather than requiring him to stop to take a reading.
One major improvement in excavator systems is the use of a laser receiver mounted on the dipperstick of the excavator, in which the laser receiver intercepts the pulsed plane of laser light that is emitted by a rotating laser light source. Naturally, the more accurate the laser receiver, the greater the possible accuracy of the operation of the excavator. Therefore, a key element of many excavator systems is the ability of the laser receiver to operate with acceptable accuracy and in varying lighting conditions. To accomplish this function, a laser receiver, generally designated by the reference numeral 30, is mounted on the dipperstick 14 of the machine 10 to accurately measure the position of the laser beam striking the receiver. The laser receiver includes a photocell assembly that is sensitive to the wavelength of light that is transmitted by a rotating laser light transmitter.
There are several products available on the market today which attempt to solve the problem of digging to a given depth, however, with varying degrees of success. One such product is called the DEPTHMASTER.TM., and is manufactured by Laser Alignment of Grand Rapids, Mich. The DEPTHMASTER is described in U.S. Pat. No. 4,884,939, invented by Nielsen. In this product, a laser receiving sensor is integrated with an inclinometer and is mounted on the dipperstick of a digging machine (e.g., an excavator). The inclinometer consists of two mercury switches configured to indicate when the angle of the stick is vertical (with respect to gravity). In operation, the combination integrated sensor informs the excavator's operator of the relative elevation of the dipperstick and whether the dipperstick is plumb. Only when the dipperstick is plumb can an accurate elevation reading be taken. The disadvantages of this approach are that the operator must stop digging to take a reading and that a reading can only be made while the dipperstick is in a plumb position. This greatly limits the practical usefulness of such the DEPTHMASTER system.
Another system for controlling excavators is known as an EXCAVATOR TOUCH SERIES 5.TM., manufactured by Topcon Laser Systems of Pleasanton, Calif. The EXCAVATOR TOUCH SERIES 5 is described in U.S. Pat. No. 4,129,224, invented by Teach. This Topcon system comprises a precision angle measuring device mounted on each of the three moving joints on the excavator, and a level sensing device mounted on the cab of the machine. In operation, the Topcon system is programmed with the dimensions of each of the machine arms such that, by using the sensor inputs and simple trigonometry, the elevation of the bucket teeth can be calculated. This calculation can be done both while the machine is stationary (static mode) as well as while in motion (dynamic mode). In practice, this is a very expensive system and is very difficult to install. As such, its market is typically limited to the very high end excavators and customers who will use the system for the majority of the machine's physical operations. Furthermore, this Topcon system is purely based on relative elevation and has no absolute elevation reference. This means that each time the machine is used, or if it is moved, a new elevation reference must be established. This is time consuming and allows for more possibility of errors during the necessary repeated setups. If an absolute elevation reference is desired, then an (additional) accessory laser system must be added. Finally, both the laser transmitter and the cab's laser receiver must be set to precisely the same angle; otherwise the resultant ditch will have a staircase effect (instead a smooth slope).
Other excavator systems are available from several manufacturers based on U.S. Pat. No. 4,491,927 (by Bachmann) in which an inclinometer is mounted on each of the boom, dipperstick, and bucket. From the physical dimensions of the Bachmann excavator and the angle of each member with respect to horizontal, trigonometry can be used to find the elevation of the bucket teeth. However, this Bachmann system also has a characteristic in which it provides relative elevation indications only, similar to the Topcon system. Furthermore, due to problems involving damping of the inclinometers, it may be impossible to provide a Bachmann system with the required accuracy while operating in a dynamic mode.
Therefore, there is a need for an elevation indication system for excavators which is low cost, easy to install, and is based on an absolute elevation reference. In addition, there is a need for an excavator indication system that provides elevation information while the operator is actually digging in the dynamic mode, rather than requiring him stop to take a reading.
Furthermore, a system that also uses the laser plane as an angular reference is inherently superior to the currently available conventional systems that use a machine mounted level reference device where the motion of the machine can cause significant errors in the sensing mechanism. In the conventional systems, when a sloped ditch is being dug (as is often the case) and a laser is used for the elevation reference and a gravity based device is used for an angular reference, it is necessary to set both the laser and the digging control system to the desired ditch slope in order to avoid a ditch bottom which has a "stair step" profile. This complicates system setup and allows for additional setup errors.